The Indo-European language family was discovered by Sir
William Jones, who noted resemblances among Greek, Latin, Sanskrit,
Germanic, and Celtic languages. He hypothesized an ancestral language
that long ago gave rise to languages in these groups. This
hypothetical, but strongly evidenced language, is called Proto-Indo-European.

The first stage of Indo-European study was the broad classification work
that established many of the well-accepted groups of Indo-European
languages.
This was done by the 1830s.
Since then, a few other languages of the family have been
added. The work of subgrouping is more difficult and there are still
points of disagreement among scholars.

In the 1850s, scholars began to reconstruct sounds and words of the
presumed ancestral language from which all Indo-European languages are
descended. This reconstruction of Proto-Indo-European is necesssarily
partial. The actual language was a normal language with tens of
thousands of vocabulary items and a full grammar, but all that can be
reconstructed of it is a few thousand words and some basic grammatical
properties.

The original homeland of the speakers of Proto-Indo-European (PIE)
is not known for certain, but many scholars believe it lies somewhere
around the Black Sea. Most of the subgroups diverged and spread out
over much of Europe and the Near East and northern Indian subcontinent
during the fourth and third millennia BC.

Since that time, enormous amounts of work have been
done on the
structure and vocabulary. The ancient Indo-European languages preserved in
writing, namely Greek, Latin, and Sanskrit, are inflected
languages. The inflections can be used to reconstruct ancient
inflections, and it is commonly accepted that Proto-Indo-European had
a number of classes of nouns and a case system slightly more elaborate
than that of Latin and Greek. The case and gender systems are said to
be best preserved, among the modern I.E. languages, in Lithuanian.

Discussion of PIE culture in the 20th century was stalled by its association with
the racist doctrines of National Socialism. But recent scholarship has
returned to the question based on archaeological finds (Gimbutas) and
the history of agriculture (Renfrew).

As PIE is not directly attested, all PIE sounds and words are
reconstructed (using the comparative
method). The standard convention is to mark
reconstructed (and therefore more or less
hypothetical) forms with an asterisk,
e.g. *wodr 'water', *k^wo:n 'dog',
*trejes 'three (masculine)', etc. Many of the
words in the modern Indo-European languages
are derived from such "protowords"
via regular sound change.

One example
of such regular sound change is Grimm's Law,
discovered about 1820 by Jakob Grimm, of
fairy-tale fame. It establishes a set of
regular correspondences between early Germanic
stops and fricatives and the stop consonants
of certain other Indo-European languages. As
formulated nowadays, Grimm's Law describes the
development of inherited Proto-Indo-European
(PIE) stops in Proto-Germanic (PGmc, the
common ancestor of the Germanic branch of the
Indo-European family). It consists of three
parts: